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revision 456 by ph10, Fri Oct 2 08:53:31 2009 UTC revision 836 by ph10, Wed Dec 28 17:16:11 2011 UTC
# Line 21  published by O'Reilly, covers regular ex Line 21  published by O'Reilly, covers regular ex
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this,  there is now also support for UTF-8 character strings. To use this,
25  PCRE must be built to include UTF-8 support, and you must call  PCRE must be built to include UTF-8 support, and you must call
26  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  is also a special sequence that can be given at the start of a pattern:  is also a special sequence that can be given at the start of a pattern:
# Line 32  Starting a pattern with this sequence is Line 32  Starting a pattern with this sequence is
32  option. This feature is not Perl-compatible. How setting UTF-8 mode affects  option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33  pattern matching is mentioned in several places below. There is also a summary  pattern matching is mentioned in several places below. There is also a summary
34  of UTF-8 features in the  of UTF-8 features in the
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
 .\"  
 in the main  
35  .\" HREF  .\" HREF
36  \fBpcre\fP  \fBpcreunicode\fP
37  .\"  .\"
38  page.  page.
39  .P  .P
40    Another special sequence that may appear at the start of a pattern or in
41    combination with (*UTF8) is:
42    .sp
43      (*UCP)
44    .sp
45    This has the same effect as setting the PCRE_UCP option: it causes sequences
46    such as \ed and \ew to use Unicode properties to determine character types,
47    instead of recognizing only characters with codes less than 128 via a lookup
48    table.
49    .P
50    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
51    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
52    also some more of these special sequences that are concerned with the handling
53    of newlines; they are described below.
54    .P
55  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
56  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
# Line 56  discussed in the Line 66  discussed in the
66  page.  page.
67  .  .
68  .  .
69    .\" HTML <a name="newlines"></a>
70  .SH "NEWLINE CONVENTIONS"  .SH "NEWLINE CONVENTIONS"
71  .rs  .rs
72  .sp  .sp
# Line 83  string with one of the following five se Line 94  string with one of the following five se
94    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
95    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
96  .sp  .sp
97  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to \fBpcre_compile()\fP or
98  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
99  newline sequence, the pattern  newline sequence, the pattern
100  .sp  .sp
# Line 95  Perl-compatible, are recognized only at Line 106  Perl-compatible, are recognized only at
106  they must be in upper case. If more than one of them is present, the last one  they must be in upper case. If more than one of them is present, the last one
107  is used.  is used.
108  .P  .P
109  The newline convention does not affect what the \eR escape sequence matches. By  The newline convention affects the interpretation of the dot metacharacter when
110  default, this is any Unicode newline sequence, for Perl compatibility. However,  PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
111  this can be changed; see the description of \eR in the section entitled  affect what the \eR escape sequence matches. By default, this is any Unicode
112    newline sequence, for Perl compatibility. However, this can be changed; see the
113    description of \eR in the section entitled
114  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
115  .\" </a>  .\" </a>
116  "Newline sequences"  "Newline sequences"
# Line 169  The following sections describe the use Line 182  The following sections describe the use
182  .rs  .rs
183  .sp  .sp
184  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
185  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
186  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
187  outside character classes.  both inside and outside character classes.
188  .P  .P
189  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
190  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 179  otherwise be interpreted as a metacharac Line 192  otherwise be interpreted as a metacharac
192  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
193  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
194  .P  .P
195    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
196    backslash. All other characters (in particular, those whose codepoints are
197    greater than 127) are treated as literals.
198    .P
199  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
200  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
201  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 198  Perl, $ and @ cause variable interpolati Line 215  Perl, $ and @ cause variable interpolati
215    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
216  .sp  .sp
217  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
218    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
219    by \eE later in the pattern, the literal interpretation continues to the end of
220    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
221    a character class, this causes an error, because the character class is not
222    terminated.
223  .  .
224  .  .
225  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 211  but when a pattern is being prepared by Line 233  but when a pattern is being prepared by
233  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it represents:
234  .sp  .sp
235    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
236    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
237    \ee        escape (hex 1B)    \ee        escape (hex 1B)
238    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
239    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
240    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
241    \et        tab (hex 09)    \et        tab (hex 09)
242    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
243    \exhh      character with hex code hh    \exhh      character with hex code hh
244    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
245      \euhhhh    character with hex code hhhh (JavaScript mode only)
246  .sp  .sp
247  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
248  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
249  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
250  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
251  .P  than 127, a compile-time error occurs. This locks out non-ASCII characters in
252  After \ex, from zero to two hexadecimal digits are read (letters can be in  both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
253  upper or lower case). Any number of hexadecimal digits may appear between \ex{  values are valid. A lower case letter is converted to upper case, and then the
254  and }, but the value of the character code must be less than 256 in non-UTF-8  0xc0 bits are flipped.)
255  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  .P
256  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  By default, after \ex, from zero to two hexadecimal digits are read (letters
257  point, which is 10FFFF.  can be in upper or lower case). Any number of hexadecimal digits may appear
258    between \ex{ and }, but the value of the character code must be less than 256
259    in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum
260    value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest
261    Unicode code point, which is 10FFFF.
262  .P  .P
263  If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
264  there is no terminating }, this form of escape is not recognized. Instead, the  there is no terminating }, this form of escape is not recognized. Instead, the
265  initial \ex will be interpreted as a basic hexadecimal escape, with no  initial \ex will be interpreted as a basic hexadecimal escape, with no
266  following digits, giving a character whose value is zero.  following digits, giving a character whose value is zero.
267  .P  .P
268    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
269    as just described only when it is followed by two hexadecimal digits.
270    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
271    code points greater than 256 is provided by \eu, which must be followed by
272    four hexadecimal digits; otherwise it matches a literal "u" character.
273    .P
274  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
275  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
276  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
277    \eu00dc in JavaScript mode).
278  .P  .P
279  After \e0 up to two further octal digits are read. If there are fewer than two  After \e0 up to two further octal digits are read. If there are fewer than two
280  digits, just those that are present are used. Thus the sequence \e0\ex\e07  digits, just those that are present are used. Thus the sequence \e0\ex\e07
# Line 294  Note that octal values of 100 or greater Line 328  Note that octal values of 100 or greater
328  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
329  .P  .P
330  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
331  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
332  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
333  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
334  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
335  meanings  inside a character class. Like other unrecognized escape sequences, they are
336  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
337  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
338  (see below).  sequences have different meanings.
339  .\"  .
340    .
341    .SS "Unsupported escape sequences"
342    .rs
343    .sp
344    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
345    handler and used to modify the case of following characters. By default, PCRE
346    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
347    option is set, \eU matches a "U" character, and \eu can be used to define a
348    character by code point, as described in the previous section.
349  .  .
350  .  .
351  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 333  syntax for referencing a subpattern as a Line 376  syntax for referencing a subpattern as a
376  later.  later.
377  .\"  .\"
378  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
379  synonymous. The former is a back reference; the latter is a  synonymous. The former is a back reference; the latter is a
380  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
381  .\" </a>  .\" </a>
382  subroutine  subroutine
# Line 341  subroutine Line 384  subroutine
384  call.  call.
385  .  .
386  .  .
387    .\" HTML <a name="genericchartypes"></a>
388  .SS "Generic character types"  .SS "Generic character types"
389  .rs  .rs
390  .sp  .sp
391  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
392  .sp  .sp
393    \ed     any decimal digit    \ed     any decimal digit
394    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
# Line 358  following are always recognized: Line 401  following are always recognized:
401    \ew     any "word" character    \ew     any "word" character
402    \eW     any "non-word" character    \eW     any "non-word" character
403  .sp  .sp
404  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
405  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
406  .P  .\" HTML <a href="#fullstopdot">
407  These character type sequences can appear both inside and outside character  .\" </a>
408    the "." metacharacter
409    .\"
410    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
411    PCRE does not support this.
412    .P
413    Each pair of lower and upper case escape sequences partitions the complete set
414    of characters into two disjoint sets. Any given character matches one, and only
415    one, of each pair. The sequences can appear both inside and outside character
416  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
417  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
418  there is no character to match.  there is no character to match.
419  .P  .P
420  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
# Line 372  are HT (9), LF (10), FF (12), CR (13), a Line 423  are HT (9), LF (10), FF (12), CR (13), a
423  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
424  does.  does.
425  .P  .P
426  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  A "word" character is an underscore or any character that is a letter or digit.
427  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  By default, the definition of letters and digits is controlled by PCRE's
428  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
429  meanings from before UTF-8 support was available, mainly for efficiency  place (see
430  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" HTML <a href="pcreapi.html#localesupport">
431  and \eW.  .\" </a>
432  .P  "Locale support"
433  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  .\"
434  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  in the
435  The horizontal space characters are:  .\" HREF
436    \fBpcreapi\fP
437    .\"
438    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
439    or "french" in Windows, some character codes greater than 128 are used for
440    accented letters, and these are then matched by \ew. The use of locales with
441    Unicode is discouraged.
442    .P
443    By default, in UTF-8 mode, characters with values greater than 128 never match
444    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
445    their original meanings from before UTF-8 support was available, mainly for
446    efficiency reasons. However, if PCRE is compiled with Unicode property support,
447    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
448    properties are used to determine character types, as follows:
449    .sp
450      \ed  any character that \ep{Nd} matches (decimal digit)
451      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
452      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
453    .sp
454    The upper case escapes match the inverse sets of characters. Note that \ed
455    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
456    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
457    \eB because they are defined in terms of \ew and \eW. Matching these sequences
458    is noticeably slower when PCRE_UCP is set.
459    .P
460    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
461    release 5.10. In contrast to the other sequences, which match only ASCII
462    characters by default, these always match certain high-valued codepoints in
463    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
464    are:
465  .sp  .sp
466    U+0009     Horizontal tab    U+0009     Horizontal tab
467    U+0020     Space    U+0020     Space
# Line 412  The vertical space characters are: Line 492  The vertical space characters are:
492    U+0085     Next line    U+0085     Next line
493    U+2028     Line separator    U+2028     Line separator
494    U+2029     Paragraph separator    U+2029     Paragraph separator
 .P  
 A "word" character is an underscore or any character less than 256 that is a  
 letter or digit. The definition of letters and digits is controlled by PCRE's  
 low-valued character tables, and may vary if locale-specific matching is taking  
 place (see  
 .\" HTML <a href="pcreapi.html#localesupport">  
 .\" </a>  
 "Locale support"  
 .\"  
 in the  
 .\" HREF  
 \fBpcreapi\fP  
 .\"  
 page). For example, in a French locale such as "fr_FR" in Unix-like systems,  
 or "french" in Windows, some character codes greater than 128 are used for  
 accented letters, and these are matched by \ew. The use of locales with Unicode  
 is discouraged.  
495  .  .
496  .  .
497  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 436  is discouraged. Line 499  is discouraged.
499  .rs  .rs
500  .sp  .sp
501  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
502  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is  Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
 equivalent to the following:  
503  .sp  .sp
504    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
505  .sp  .sp
# Line 468  one of the following sequences: Line 530  one of the following sequences:
530    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
531    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
532  .sp  .sp
533  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to \fBpcre_compile()\fP or
534  \fBpcre_compile2()\fP, but they can be overridden by options given to  \fBpcre_compile2()\fP, but they can be overridden by options given to
535  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
536  which are not Perl-compatible, are recognized only at the very start of a  which are not Perl-compatible, are recognized only at the very start of a
537  pattern, and that they must be in upper case. If more than one of them is  pattern, and that they must be in upper case. If more than one of them is
538  present, the last one is used. They can be combined with a change of newline  present, the last one is used. They can be combined with a change of newline
539  convention, for example, a pattern can start with:  convention; for example, a pattern can start with:
540  .sp  .sp
541    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
542  .sp  .sp
543  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
544    a character class, \eR is treated as an unrecognized escape sequence, and so
545    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
546  .  .
547  .  .
548  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 496  The extra escape sequences are: Line 560  The extra escape sequences are:
560    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
561  .sp  .sp
562  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
563  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
564  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
565  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
566  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
567    .\" </a>
568    next section).
569    .\"
570    Other Perl properties such as "InMusicalSymbols" are not currently supported by
571    PCRE. Note that \eP{Any} does not match any characters, so always causes a
572    match failure.
573  .P  .P
574  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
575  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 513  Those that are not part of an identified Line 583  Those that are not part of an identified
583  .P  .P
584  Arabic,  Arabic,
585  Armenian,  Armenian,
586    Avestan,
587  Balinese,  Balinese,
588    Bamum,
589  Bengali,  Bengali,
590  Bopomofo,  Bopomofo,
591  Braille,  Braille,
592  Buginese,  Buginese,
593  Buhid,  Buhid,
594  Canadian_Aboriginal,  Canadian_Aboriginal,
595    Carian,
596    Cham,
597  Cherokee,  Cherokee,
598  Common,  Common,
599  Coptic,  Coptic,
# Line 528  Cypriot, Line 602  Cypriot,
602  Cyrillic,  Cyrillic,
603  Deseret,  Deseret,
604  Devanagari,  Devanagari,
605    Egyptian_Hieroglyphs,
606  Ethiopic,  Ethiopic,
607  Georgian,  Georgian,
608  Glagolitic,  Glagolitic,
# Line 540  Hangul, Line 615  Hangul,
615  Hanunoo,  Hanunoo,
616  Hebrew,  Hebrew,
617  Hiragana,  Hiragana,
618    Imperial_Aramaic,
619  Inherited,  Inherited,
620    Inscriptional_Pahlavi,
621    Inscriptional_Parthian,
622    Javanese,
623    Kaithi,
624  Kannada,  Kannada,
625  Katakana,  Katakana,
626    Kayah_Li,
627  Kharoshthi,  Kharoshthi,
628  Khmer,  Khmer,
629  Lao,  Lao,
630  Latin,  Latin,
631    Lepcha,
632  Limbu,  Limbu,
633  Linear_B,  Linear_B,
634    Lisu,
635    Lycian,
636    Lydian,
637  Malayalam,  Malayalam,
638    Meetei_Mayek,
639  Mongolian,  Mongolian,
640  Myanmar,  Myanmar,
641  New_Tai_Lue,  New_Tai_Lue,
# Line 557  Nko, Line 643  Nko,
643  Ogham,  Ogham,
644  Old_Italic,  Old_Italic,
645  Old_Persian,  Old_Persian,
646    Old_South_Arabian,
647    Old_Turkic,
648    Ol_Chiki,
649  Oriya,  Oriya,
650  Osmanya,  Osmanya,
651  Phags_Pa,  Phags_Pa,
652  Phoenician,  Phoenician,
653    Rejang,
654  Runic,  Runic,
655    Samaritan,
656    Saurashtra,
657  Shavian,  Shavian,
658  Sinhala,  Sinhala,
659    Sundanese,
660  Syloti_Nagri,  Syloti_Nagri,
661  Syriac,  Syriac,
662  Tagalog,  Tagalog,
663  Tagbanwa,  Tagbanwa,
664  Tai_Le,  Tai_Le,
665    Tai_Tham,
666    Tai_Viet,
667  Tamil,  Tamil,
668  Telugu,  Telugu,
669  Thaana,  Thaana,
# Line 576  Thai, Line 671  Thai,
671  Tibetan,  Tibetan,
672  Tifinagh,  Tifinagh,
673  Ugaritic,  Ugaritic,
674    Vai,
675  Yi.  Yi.
676  .P  .P
677  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
678  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
679  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
680  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
681  .P  .P
682  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
683  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 677  Characters with the "mark" property are Line 773  Characters with the "mark" property are
773  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
774  non-UTF-8 mode \eX matches any one character.  non-UTF-8 mode \eX matches any one character.
775  .P  .P
776    Note that recent versions of Perl have changed \eX to match what Unicode calls
777    an "extended grapheme cluster", which has a more complicated definition.
778    .P
779  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
780  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
781  why the traditional escape sequences such as \ed and \ew do not use Unicode  why the traditional escape sequences such as \ed and \ew do not use Unicode
782  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
783    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
784    (*UCP).
785    .
786    .
787    .\" HTML <a name="extraprops"></a>
788    .SS PCRE's additional properties
789    .rs
790    .sp
791    As well as the standard Unicode properties described in the previous
792    section, PCRE supports four more that make it possible to convert traditional
793    escape sequences such as \ew and \es and POSIX character classes to use Unicode
794    properties. PCRE uses these non-standard, non-Perl properties internally when
795    PCRE_UCP is set. They are:
796    .sp
797      Xan   Any alphanumeric character
798      Xps   Any POSIX space character
799      Xsp   Any Perl space character
800      Xwd   Any Perl "word" character
801    .sp
802    Xan matches characters that have either the L (letter) or the N (number)
803    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
804    carriage return, and any other character that has the Z (separator) property.
805    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
806    same characters as Xan, plus underscore.
807  .  .
808  .  .
809  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
810  .SS "Resetting the match start"  .SS "Resetting the match start"
811  .rs  .rs
812  .sp  .sp
813  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
814  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
815  .sp  .sp
816    foo\eKbar    foo\eKbar
817  .sp  .sp
# Line 711  For example, when the pattern Line 833  For example, when the pattern
833    (foo)\eKbar    (foo)\eKbar
834  .sp  .sp
835  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
836    .P
837    Perl documents that the use of \eK within assertions is "not well defined". In
838    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
839    ignored in negative assertions.
840  .  .
841  .  .
842  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 735  The backslashed assertions are: Line 861  The backslashed assertions are:
861    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
862    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
863  .sp  .sp
864  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
865  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
866    default it matches the corresponding literal character (for example, \eB
867    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
868    escape sequence" error is generated instead.
869  .P  .P
870  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
871  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
872  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
873  first or last character matches \ew, respectively. Neither PCRE nor Perl has a  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
874  separte "start of word" or "end of word" metasequence. However, whatever  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
875  follows \eb normally determines which it is. For example, the fragment  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
876  \eba matches "a" at the start of a word.  of word" or "end of word" metasequence. However, whatever follows \eb normally
877    determines which it is. For example, the fragment \eba matches "a" at the start
878    of a word.
879  .P  .P
880  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
881  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 828  end of the subject in both modes, and if Line 959  end of the subject in both modes, and if
959  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
960  .  .
961  .  .
962  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
963    .SH "FULL STOP (PERIOD, DOT) AND \eN"
964  .rs  .rs
965  .sp  .sp
966  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
# Line 850  to match it. Line 982  to match it.
982  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
983  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
984  special meaning in a character class.  special meaning in a character class.
985    .P
986    The escape sequence \eN behaves like a dot, except that it is not affected by
987    the PCRE_DOTALL option. In other words, it matches any character except one
988    that signifies the end of a line. Perl also uses \eN to match characters by
989    name; PCRE does not support this.
990  .  .
991  .  .
992  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
993  .rs  .rs
994  .sp  .sp
995  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
996  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  in and out of UTF-8 mode. Unlike a dot, it always matches line-ending
997  characters. The feature is provided in Perl in order to match individual bytes  characters. The feature is provided in Perl in order to match individual bytes
998  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  in UTF-8 mode, but it is unclear how it can usefully be used. Because \eC
999  what remains in the string may be a malformed UTF-8 string. For this reason,  breaks up characters into individual bytes, matching one byte with \eC in UTF-8
1000  the \eC escape sequence is best avoided.  mode means that the rest of the string may start with a malformed UTF-8
1001    character. This has undefined results, because PCRE assumes that it is dealing
1002    with valid UTF-8 strings (and by default it checks this at the start of
1003    processing unless the PCRE_NO_UTF8_CHECK option is used).
1004  .P  .P
1005  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1006  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1007  .\" </a>  .\" </a>
1008  (described below),  (described below)
1009  .\"  .\"
1010  because in UTF-8 mode this would make it impossible to calculate the length of  in UTF-8 mode, because this would make it impossible to calculate the length of
1011  the lookbehind.  the lookbehind.
1012    .P
1013    In general, the \eC escape sequence is best avoided in UTF-8 mode. However, one
1014    way of using it that avoids the problem of malformed UTF-8 characters is to
1015    use a lookahead to check the length of the next character, as in this pattern
1016    (ignore white space and line breaks):
1017    .sp
1018      (?| (?=[\ex00-\ex7f])(\eC) |
1019          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1020          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1021          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1022    .sp
1023    A group that starts with (?| resets the capturing parentheses numbers in each
1024    alternative (see
1025    .\" HTML <a href="#dupsubpatternnumber">
1026    .\" </a>
1027    "Duplicate Subpattern Numbers"
1028    .\"
1029    below). The assertions at the start of each branch check the next UTF-8
1030    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1031    character's individual bytes are then captured by the appropriate number of
1032    groups.
1033  .  .
1034  .  .
1035  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 876  the lookbehind. Line 1037  the lookbehind.
1037  .rs  .rs
1038  .sp  .sp
1039  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1040  square bracket. A closing square bracket on its own is not special by default.  square bracket. A closing square bracket on its own is not special by default.
1041  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1042  bracket causes a compile-time error. If a closing square bracket is required as  bracket causes a compile-time error. If a closing square bracket is required as
1043  a member of the class, it should be the first data character in the class  a member of the class, it should be the first data character in the class
1044  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
# Line 944  characters in both cases. In UTF-8 mode, Line 1105  characters in both cases. In UTF-8 mode,
1105  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1106  property support.  property support.
1107  .P  .P
1108  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1109  in a character class, and add the characters that they match to the class. For  \eV, \ew, and \eW may appear in a character class, and add the characters that
1110  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1111  conveniently be used with the upper case character types to specify a more  digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1112  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1113  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1114    .\" HTML <a href="#genericchartypes">
1115    .\" </a>
1116    "Generic character types"
1117    .\"
1118    above. The escape sequence \eb has a different meaning inside a character
1119    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1120    are not special inside a character class. Like any other unrecognized escape
1121    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1122    default, but cause an error if the PCRE_EXTRA option is set.
1123    .P
1124    A circumflex can conveniently be used with the upper case character types to
1125    specify a more restricted set of characters than the matching lower case type.
1126    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1127    whereas [\ew] includes underscore. A positive character class should be read as
1128    "something OR something OR ..." and a negative class as "NOT something AND NOT
1129    something AND NOT ...".
1130  .P  .P
1131  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1132  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 969  this notation. For example, Line 1146  this notation. For example,
1146    [01[:alpha:]%]    [01[:alpha:]%]
1147  .sp  .sp
1148  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1149  are  are:
1150  .sp  .sp
1151    alnum    letters and digits    alnum    letters and digits
1152    alpha    letters    alpha    letters
# Line 980  are Line 1157  are
1157    graph    printing characters, excluding space    graph    printing characters, excluding space
1158    lower    lower case letters    lower    lower case letters
1159    print    printing characters, including space    print    printing characters, including space
1160    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1161    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1162    upper    upper case letters    upper    upper case letters
1163    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 1001  matches "1", "2", or any non-digit. PCRE Line 1178  matches "1", "2", or any non-digit. PCRE
1178  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1179  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1180  .P  .P
1181  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF-8 mode, characters with values greater than 128 do not match
1182  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1183    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1184    character properties are used. This is achieved by replacing the POSIX classes
1185    by other sequences, as follows:
1186    .sp
1187      [:alnum:]  becomes  \ep{Xan}
1188      [:alpha:]  becomes  \ep{L}
1189      [:blank:]  becomes  \eh
1190      [:digit:]  becomes  \ep{Nd}
1191      [:lower:]  becomes  \ep{Ll}
1192      [:space:]  becomes  \ep{Xps}
1193      [:upper:]  becomes  \ep{Lu}
1194      [:word:]   becomes  \ep{Xwd}
1195    .sp
1196    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1197    classes are unchanged, and match only characters with code points less than
1198    128.
1199  .  .
1200  .  .
1201  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1056  extracts it into the global options (and Line 1249  extracts it into the global options (and
1249  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1250  .P  .P
1251  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1252  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1253  .sp  .sp
1254    (a(?i)b)c    (a(?i)b)c
1255  .sp  .sp
# Line 1081  section entitled Line 1274  section entitled
1274  .\" </a>  .\" </a>
1275  "Newline sequences"  "Newline sequences"
1276  .\"  .\"
1277  above. There is also the (*UTF8) leading sequence that can be used to set UTF-8  above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1278  mode; this is equivalent to setting the PCRE_UTF8 option.  to set UTF-8 and Unicode property modes; they are equivalent to setting the
1279    PCRE_UTF8 and the PCRE_UCP options, respectively.
1280  .  .
1281  .  .
1282  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1096  Turning part of a pattern into a subpatt Line 1290  Turning part of a pattern into a subpatt
1290  .sp  .sp
1291    cat(aract|erpillar|)    cat(aract|erpillar|)
1292  .sp  .sp
1293  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1294  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1295  .sp  .sp
1296  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1297  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
1298  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
1299  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1300  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1301  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1302  .sp  .sp
1303    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1304  .sp  .sp
# Line 1154  at captured substring number one, whiche Line 1347  at captured substring number one, whiche
1347  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1348  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1349  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1350  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1351  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1352  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1353  .sp  .sp
1354    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1355    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1356    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1357  .sp  .sp
1358  A backreference to a numbered subpattern uses the most recent value that is set  A back reference to a numbered subpattern uses the most recent value that is
1359  for that number by any subpattern. The following pattern matches "abcabc" or  set for that number by any subpattern. The following pattern matches "abcabc"
1360  "defdef":  or "defdef":
1361  .sp  .sp
1362    /(?|(abc)|(def))\1/    /(?|(abc)|(def))\e1/
1363  .sp  .sp
1364  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a subroutine call to a numbered subpattern always refers to the
1365  refers to the first one in the pattern with the given number. The following  first one in the pattern with the given number. The following pattern matches
1366  pattern matches "abcabc" or "defabc":  "abcabc" or "defabc":
1367  .sp  .sp
1368    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1369  .sp  .sp
1370    If a
1371    .\" HTML <a href="#conditions">
1372    .\" </a>
1373    condition test
1374    .\"
1375    for a subpattern's having matched refers to a non-unique number, the test is
1376    true if any of the subpatterns of that number have matched.
1377  .P  .P
1378  An alternative approach to using the "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1379  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
1380  .  .
1381  .  .
# Line 1189  if an expression is modified, the number Line 1388  if an expression is modified, the number
1388  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1389  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1390  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1391  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1392    have different names, but PCRE does not.
1393  .P  .P
1394  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1395  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1396  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1397  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1398  .\" </a>  .\" </a>
1399  backreferences,  back references,
1400  .\"  .\"
1401  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1402  .\" </a>  .\" </a>
# Line 1216  extracting the name-to-number translatio Line 1416  extracting the name-to-number translatio
1416  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1417  .P  .P
1418  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
1419  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1420  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1421  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1422  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1423  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1424    name, and in both cases you want to extract the abbreviation. This pattern
1425    (ignoring the line breaks) does the job:
1426  .sp  .sp
1427    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1428    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1234  subpattern, as described in the previous Line 1436  subpattern, as described in the previous
1436  .P  .P
1437  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1438  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1439  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1440  make a reference to a non-unique named subpattern from elsewhere in the  .P
1441  pattern, the one that corresponds to the lowest number is used. For further  If you make a back reference to a non-unique named subpattern from elsewhere in
1442  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1443    used. In the absence of duplicate numbers (see the previous section) this is
1444    the one with the lowest number. If you use a named reference in a condition
1445    test (see the
1446    .\"
1447    .\" HTML <a href="#conditions">
1448    .\" </a>
1449    section about conditions
1450    .\"
1451    below), either to check whether a subpattern has matched, or to check for
1452    recursion, all subpatterns with the same name are tested. If the condition is
1453    true for any one of them, the overall condition is true. This is the same
1454    behaviour as testing by number. For further details of the interfaces for
1455    handling named subpatterns, see the
1456  .\" HREF  .\" HREF
1457  \fBpcreapi\fP  \fBpcreapi\fP
1458  .\"  .\"
1459  documentation.  documentation.
1460  .P  .P
1461  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1462  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1463  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1464    are given to subpatterns with the same number. However, you can give the same
1465    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1466  .  .
1467  .  .
1468  .SH REPETITION  .SH REPETITION
# Line 1259  items: Line 1476  items:
1476    the \eC escape sequence    the \eC escape sequence
1477    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1478    the \eR escape sequence    the \eR escape sequence
1479    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1480    a character class    a character class
1481    a back reference (see next section)    a back reference (see next section)
1482    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1483    a recursive or "subroutine" call to a subpattern    a subroutine call to a subpattern (recursive or otherwise)
1484  .sp  .sp
1485  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1486  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1301  subpatterns that are referenced as Line 1518  subpatterns that are referenced as
1518  .\" </a>  .\" </a>
1519  subroutines  subroutines
1520  .\"  .\"
1521  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1522  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1523    .\" </a>
1524    "Defining subpatterns for use by reference only"
1525    .\"
1526    below). Items other than subpatterns that have a {0} quantifier are omitted
1527    from the compiled pattern.
1528  .P  .P
1529  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1530  abbreviations:  abbreviations:
# Line 1374  worth setting PCRE_DOTALL in order to ob Line 1596  worth setting PCRE_DOTALL in order to ob
1596  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1597  .P  .P
1598  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1599  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1600  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1601  succeeds. Consider, for example:  succeeds. Consider, for example:
1602  .sp  .sp
# Line 1527  no such problem when named parentheses a Line 1749  no such problem when named parentheses a
1749  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1750  .P  .P
1751  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1752  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence. This escape must be followed by an
1753  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1754  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1755  .sp  .sp
1756    (ring), \e1    (ring), \e1
1757    (ring), \eg1    (ring), \eg1
# Line 1543  example: Line 1765  example:
1765    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1766  .sp  .sp
1767  The sequence \eg{-1} is a reference to the most recently started capturing  The sequence \eg{-1} is a reference to the most recently started capturing
1768  subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}  subpattern before \eg, that is, is it equivalent to \e2 in this example.
1769  would be equivalent to \e1. The use of relative references can be helpful in  Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1770  long patterns, and also in patterns that are created by joining together  can be helpful in long patterns, and also in patterns that are created by
1771  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1772  .P  .P
1773  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1774  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1589  references to it always fail by default. Line 1811  references to it always fail by default.
1811  .sp  .sp
1812    (a|(bc))\e2    (a|(bc))\e2
1813  .sp  .sp
1814  always fails if it starts to match "a" rather than "bc". However, if the  always fails if it starts to match "a" rather than "bc". However, if the
1815  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1816  unset value matches an empty string.  unset value matches an empty string.
1817  .P  .P
1818  Because there may be many capturing parentheses in a pattern, all digits  Because there may be many capturing parentheses in a pattern, all digits
# Line 1603  whitespace. Otherwise, the \eg{ syntax o Line 1825  whitespace. Otherwise, the \eg{ syntax o
1825  "Comments"  "Comments"
1826  .\"  .\"
1827  below) can be used.  below) can be used.
1828  .P  .
1829    .SS "Recursive back references"
1830    .rs
1831    .sp
1832  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1833  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1834  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1617  to the previous iteration. In order for Line 1842  to the previous iteration. In order for
1842  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1843  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1844  minimum of zero.  minimum of zero.
1845    .P
1846    Back references of this type cause the group that they reference to be treated
1847    as an
1848    .\" HTML <a href="#atomicgroup">
1849    .\" </a>
1850    atomic group.
1851    .\"
1852    Once the whole group has been matched, a subsequent matching failure cannot
1853    cause backtracking into the middle of the group.
1854  .  .
1855  .  .
1856  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1636  those that look ahead of the current pos Line 1870  those that look ahead of the current pos
1870  that look behind it. An assertion subpattern is matched in the normal way,  that look behind it. An assertion subpattern is matched in the normal way,
1871  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
1872  .P  .P
1873  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1874  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
1875  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1876  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1877  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1878  because it does not make sense for negative assertions.  .P
1879    For compatibility with Perl, assertion subpatterns may be repeated; though
1880    it makes no sense to assert the same thing several times, the side effect of
1881    capturing parentheses may occasionally be useful. In practice, there only three
1882    cases:
1883    .sp
1884    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1885    However, it may contain internal capturing parenthesized groups that are called
1886    from elsewhere via the
1887    .\" HTML <a href="#subpatternsassubroutines">
1888    .\" </a>
1889    subroutine mechanism.
1890    .\"
1891    .sp
1892    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1893    were {0,1}. At run time, the rest of the pattern match is tried with and
1894    without the assertion, the order depending on the greediness of the quantifier.
1895    .sp
1896    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1897    The assertion is obeyed just once when encountered during matching.
1898  .  .
1899  .  .
1900  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1670  lookbehind assertion is needed to achiev Line 1923  lookbehind assertion is needed to achiev
1923  If you want to force a matching failure at some point in a pattern, the most  If you want to force a matching failure at some point in a pattern, the most
1924  convenient way to do it is with (?!) because an empty string always matches, so  convenient way to do it is with (?!) because an empty string always matches, so
1925  an assertion that requires there not to be an empty string must always fail.  an assertion that requires there not to be an empty string must always fail.
1926  The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a  The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
 synonym for (?!).  
1927  .  .
1928  .  .
1929  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1696  is permitted, but Line 1948  is permitted, but
1948  .sp  .sp
1949  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1950  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1951  extension compared with Perl (5.8 and 5.10), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1952  match the same length of string. An assertion such as  length of string. An assertion such as
1953  .sp  .sp
1954    (?<=ab(c|de))    (?<=ab(c|de))
1955  .sp  .sp
# Line 1707  branches: Line 1959  branches:
1959  .sp  .sp
1960    (?<=abc|abde)    (?<=abc|abde)
1961  .sp  .sp
1962  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1963  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1964  .\" </a>  .\" </a>
1965  (see above)  (see above)
1966  .\"  .\"
1967  can be used instead of a lookbehind assertion to get round the fixed-length  can be used instead of a lookbehind assertion to get round the fixed-length
1968  restriction.  restriction.
1969  .P  .P
1970  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
# Line 1720  temporarily move the current position ba Line 1972  temporarily move the current position ba
1972  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1973  assertion fails.  assertion fails.
1974  .P  .P
1975  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  In UTF-8 mode, PCRE does not allow the \eC escape (which matches a single byte,
1976  to appear in lookbehind assertions, because it makes it impossible to calculate  even in UTF-8 mode) to appear in lookbehind assertions, because it makes it
1977  the length of the lookbehind. The \eX and \eR escapes, which can match  impossible to calculate the length of the lookbehind. The \eX and \eR escapes,
1978  different numbers of bytes, are also not permitted.  which can match different numbers of bytes, are also not permitted.
1979  .P  .P
1980  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
1981  .\" </a>  .\" </a>
1982  "Subroutine"  "Subroutine"
1983  .\"  .\"
1984  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1985  as the subpattern matches a fixed-length string.  as the subpattern matches a fixed-length string.
1986  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1987  .\" </a>  .\" </a>
1988  Recursion,  Recursion,
# Line 1803  characters that are not "999". Line 2055  characters that are not "999".
2055  .sp  .sp
2056  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2057  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2058  the result of an assertion, or whether a specific capturing subpattern has  the result of an assertion, or whether a specific capturing subpattern has
2059  already been matched. The two possible forms of conditional subpattern are:  already been matched. The two possible forms of conditional subpattern are:
2060  .sp  .sp
2061    (?(condition)yes-pattern)    (?(condition)yes-pattern)
# Line 1811  already been matched. The two possible f Line 2063  already been matched. The two possible f
2063  .sp  .sp
2064  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2065  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
2066  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2067    itself contain nested subpatterns of any form, including conditional
2068    subpatterns; the restriction to two alternatives applies only at the level of
2069    the condition. This pattern fragment is an example where the alternatives are
2070    complex:
2071    .sp
2072      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2073    .sp
2074  .P  .P
2075  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2076  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1821  recursion, a pseudo-condition called DEF Line 2080  recursion, a pseudo-condition called DEF
2080  .sp  .sp
2081  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
2082  condition is true if a capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2083  matched. If there is more than one capturing subpattern with the same number  matched. If there is more than one capturing subpattern with the same number
2084  (see the earlier  (see the earlier
2085  .\"  .\"
2086  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
2087  .\" </a>  .\" </a>
2088  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2089  .\"  .\"
2090  the condition is true if any of them have been set. An alternative notation is  the condition is true if any of them have matched. An alternative notation is
2091  to precede the digits with a plus or minus sign. In this case, the subpattern  to precede the digits with a plus or minus sign. In this case, the subpattern
2092  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2093  can be referenced by (?(-1), the next most recent by (?(-2), and so on. In  can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2094  looping constructs it can also make sense to refer to subsequent groups with  loops it can also make sense to refer to subsequent groups. The next
2095  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2096    zero in any of these forms is not used; it provokes a compile-time error.)
2097  .P  .P
2098  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2099  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1844  three parts for ease of discussion: Line 2104  three parts for ease of discussion:
2104  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2105  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
2106  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
2107  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2108  or not. If they did, that is, if subject started with an opening parenthesis,  matched. If they did, that is, if subject started with an opening parenthesis,
2109  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
2110  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2111  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 1874  Rewriting the above example to use a nam Line 2134  Rewriting the above example to use a nam
2134  .sp  .sp
2135    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2136  .sp  .sp
2137    If the name used in a condition of this kind is a duplicate, the test is
2138    applied to all subpatterns of the same name, and is true if any one of them has
2139    matched.
2140  .  .
2141  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2142  .rs  .rs
# Line 1887  letter R, for example: Line 2150  letter R, for example:
2150  .sp  .sp
2151  the condition is true if the most recent recursion is into a subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2152  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
2153  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2154    applied to all subpatterns of the same name, and is true if any one of them is
2155    the most recent recursion.
2156  .P  .P
2157  At "top level", all these recursion test conditions are false.  At "top level", all these recursion test conditions are false.
2158  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
2159  .\" </a>  .\" </a>
2160  Recursive patterns  The syntax for recursive patterns
2161  .\"  .\"
2162  are described below.  is described below.
2163  .  .
2164    .\" HTML <a name="subdefine"></a>
2165  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2166  .rs  .rs
2167  .sp  .sp
# Line 1903  If the condition is the string (DEFINE), Line 2169  If the condition is the string (DEFINE),
2169  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
2170  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2171  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
2172  "subroutines" that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
2173  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2174  .\" </a>  .\" </a>
2175  "subroutines"  subroutines
2176  .\"  .\"
2177  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address such as
2178  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2179    breaks):
2180  .sp  .sp
2181    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2182    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1944  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2211  dd-aaa-dd or dd-dd-dd, where aaa are let
2211  .SH COMMENTS  .SH COMMENTS
2212  .rs  .rs
2213  .sp  .sp
2214  The sequence (?# marks the start of a comment that continues up to the next  There are two ways of including comments in patterns that are processed by
2215  closing parenthesis. Nested parentheses are not permitted. The characters  PCRE. In both cases, the start of the comment must not be in a character class,
2216  that make up a comment play no part in the pattern matching at all.  nor in the middle of any other sequence of related characters such as (?: or a
2217    subpattern name or number. The characters that make up a comment play no part
2218    in the pattern matching.
2219  .P  .P
2220  If the PCRE_EXTENDED option is set, an unescaped # character outside a  The sequence (?# marks the start of a comment that continues up to the next
2221  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2222  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2223    this case continues to immediately after the next newline character or
2224    character sequence in the pattern. Which characters are interpreted as newlines
2225    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2226    sequence at the start of the pattern, as described in the section entitled
2227    .\" HTML <a href="#newlines">
2228    .\" </a>
2229    "Newline conventions"
2230    .\"
2231    above. Note that the end of this type of comment is a literal newline sequence
2232    in the pattern; escape sequences that happen to represent a newline do not
2233    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2234    default newline convention is in force:
2235    .sp
2236      abc #comment \en still comment
2237    .sp
2238    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2239    a newline in the pattern. The sequence \en is still literal at this stage, so
2240    it does not terminate the comment. Only an actual character with the code value
2241    0x0a (the default newline) does so.
2242  .  .
2243  .  .
2244  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1979  individual subpattern recursion. After i Line 2267  individual subpattern recursion. After i
2267  this kind of recursion was subsequently introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2268  .P  .P
2269  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2270  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive subroutine call of the subpattern of the
2271  provided that it occurs inside that subpattern. (If not, it is a  given number, provided that it occurs inside that subpattern. (If not, it is a
2272  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2273  .\" </a>  .\" </a>
2274  "subroutine"  non-recursive subroutine
2275  .\"  .\"
2276  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2277  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
# Line 1996  PCRE_EXTENDED option is set so that whit Line 2284  PCRE_EXTENDED option is set so that whit
2284  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2285  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
2286  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2287  Finally there is a closing parenthesis. Note the use of a possessive quantifier  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2288  to avoid backtracking into sequences of non-parentheses.  to avoid backtracking into sequences of non-parentheses.
2289  .P  .P
2290  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
# Line 2008  We have put the pattern into parentheses Line 2296  We have put the pattern into parentheses
2296  them instead of the whole pattern.  them instead of the whole pattern.
2297  .P  .P
2298  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2299  is made easier by the use of relative references (a Perl 5.10 feature).  is made easier by the use of relative references. Instead of (?1) in the
2300  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  pattern above you can write (?-2) to refer to the second most recently opened
2301  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2302  negative number counts capturing parentheses leftwards from the point at which  capturing parentheses leftwards from the point at which it is encountered.
 it is encountered.  
2303  .P  .P
2304  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2305  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2306  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2307  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2308  .\" </a>  .\" </a>
2309  "subroutine"  non-recursive subroutine
2310  .\"  .\"
2311  calls, as described in the next section.  calls, as described in the next section.
2312  .P  .P
# Line 2044  the match runs for a very long time inde Line 2331  the match runs for a very long time inde
2331  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
2332  before failure can be reported.  before failure can be reported.
2333  .P  .P
2334  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2335  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2336  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2337  .\" HREF  .\" HREF
2338  \fBpcrecallout\fP  \fBpcrecallout\fP
2339  .\"  .\"
# Line 2055  documentation). If the pattern above is Line 2341  documentation). If the pattern above is
2341  .sp  .sp
2342    (ab(cd)ef)    (ab(cd)ef)
2343  .sp  .sp
2344  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2345  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2346  .sp  matched at the top level, its final captured value is unset, even if it was
2347    \e( ( ( [^()]++ | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2348       ^                        ^  .P
2349       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2350  .sp  obtain extra memory to store data during a recursion, which it does by using
2351  the string they capture is "ab(cd)ef", the contents of the top level  \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2352  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 has to obtain extra memory to store data during a recursion, which it does by  
 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
2353  .P  .P
2354  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
2355  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 2081  is the actual recursive call. Line 2364  is the actual recursive call.
2364  .  .
2365  .  .
2366  .\" HTML <a name="recursiondifference"></a>  .\" HTML <a name="recursiondifference"></a>
2367  .SS "Recursion difference from Perl"  .SS "Differences in recursion processing between PCRE and Perl"
2368  .rs  .rs
2369  .sp  .sp
2370  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2371  treated as an atomic group. That is, once it has matched some of the subject  (like Python, but unlike Perl), a recursive subpattern call is always treated
2372  string, it is never re-entered, even if it contains untried alternatives and  as an atomic group. That is, once it has matched some of the subject string, it
2373  there is a subsequent matching failure. This can be illustrated by the  is never re-entered, even if it contains untried alternatives and there is a
2374  following pattern, which purports to match a palindromic string that contains  subsequent matching failure. This can be illustrated by the following pattern,
2375  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  which purports to match a palindromic string that contains an odd number of
2376    characters (for example, "a", "aba", "abcba", "abcdcba"):
2377  .sp  .sp
2378    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2379  .sp  .sp
2380  The idea is that it either matches a single character, or two identical  The idea is that it either matches a single character, or two identical
2381  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2382  it does not if the pattern is longer than three characters. Consider the  it does not if the pattern is longer than three characters. Consider the
2383  subject string "abcba":  subject string "abcba":
2384  .P  .P
2385  At the top level, the first character is matched, but as it is not at the end  At the top level, the first character is matched, but as it is not at the end
2386  of the string, the first alternative fails; the second alternative is taken  of the string, the first alternative fails; the second alternative is taken
2387  and the recursion kicks in. The recursive call to subpattern 1 successfully  and the recursion kicks in. The recursive call to subpattern 1 successfully
2388  matches the next character ("b"). (Note that the beginning and end of line  matches the next character ("b"). (Note that the beginning and end of line
2389  tests are not part of the recursion).  tests are not part of the recursion).
2390  .P  .P
2391  Back at the top level, the next character ("c") is compared with what  Back at the top level, the next character ("c") is compared with what
2392  subpattern 2 matched, which was "a". This fails. Because the recursion is  subpattern 2 matched, which was "a". This fails. Because the recursion is
2393  treated as an atomic group, there are now no backtracking points, and so the  treated as an atomic group, there are now no backtracking points, and so the
2394  entire match fails. (Perl is able, at this point, to re-enter the recursion and  entire match fails. (Perl is able, at this point, to re-enter the recursion and
2395  try the second alternative.) However, if the pattern is written with the  try the second alternative.) However, if the pattern is written with the
# Line 2113  alternatives in the other order, things Line 2397  alternatives in the other order, things
2397  .sp  .sp
2398    ^((.)(?1)\e2|.)$    ^((.)(?1)\e2|.)$
2399  .sp  .sp
2400  This time, the recursing alternative is tried first, and continues to recurse  This time, the recursing alternative is tried first, and continues to recurse
2401  until it runs out of characters, at which point the recursion fails. But this  until it runs out of characters, at which point the recursion fails. But this
2402  time we do have another alternative to try at the higher level. That is the big  time we do have another alternative to try at the higher level. That is the big
2403  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2404  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2405  .P  .P
2406  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that it matches all palindromic strings, not just
2407  with an odd number of characters, it is tempting to change the pattern to this:  those with an odd number of characters, it is tempting to change the pattern to
2408    this:
2409  .sp  .sp
2410    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2411  .sp  .sp
2412  Again, this works in Perl, but not in PCRE, and for the same reason. When a  Again, this works in Perl, but not in PCRE, and for the same reason. When a
2413  deeper recursion has matched a single character, it cannot be entered again in  deeper recursion has matched a single character, it cannot be entered again in
2414  order to match an empty string. The solution is to separate the two cases, and  order to match an empty string. The solution is to separate the two cases, and
2415  write out the odd and even cases as alternatives at the higher level:  write out the odd and even cases as alternatives at the higher level:
2416  .sp  .sp
2417    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2418  .sp  .sp
2419  If you want to match typical palindromic phrases, the pattern has to ignore all  If you want to match typical palindromic phrases, the pattern has to ignore all
2420  non-word characters, which can be done like this:  non-word characters, which can be done like this:
2421  .sp  .sp
2422    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2423  .sp  .sp
2424  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2425  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2426  the use of the possessive quantifier *+ to avoid backtracking into sequences of  the use of the possessive quantifier *+ to avoid backtracking into sequences of
2427  non-word characters. Without this, PCRE takes a great deal longer (ten times or  non-word characters. Without this, PCRE takes a great deal longer (ten times or
2428  more) to match typical phrases, and Perl takes so long that you think it has  more) to match typical phrases, and Perl takes so long that you think it has
2429  gone into a loop.  gone into a loop.
# Line 2149  For example, although "abcba" is correct Line 2434  For example, although "abcba" is correct
2434  PCRE finds the palindrome "aba" at the start, then fails at top level because  PCRE finds the palindrome "aba" at the start, then fails at top level because
2435  the end of the string does not follow. Once again, it cannot jump back into the  the end of the string does not follow. Once again, it cannot jump back into the
2436  recursion to try other alternatives, so the entire match fails.  recursion to try other alternatives, so the entire match fails.
2437    .P
2438    The second way in which PCRE and Perl differ in their recursion processing is
2439    in the handling of captured values. In Perl, when a subpattern is called
2440    recursively or as a subpattern (see the next section), it has no access to any
2441    values that were captured outside the recursion, whereas in PCRE these values
2442    can be referenced. Consider this pattern:
2443    .sp
2444      ^(.)(\e1|a(?2))
2445    .sp
2446    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2447    then in the second group, when the back reference \e1 fails to match "b", the
2448    second alternative matches "a" and then recurses. In the recursion, \e1 does
2449    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2450    match because inside the recursive call \e1 cannot access the externally set
2451    value.
2452  .  .
2453  .  .
2454  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2455  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2456  .rs  .rs
2457  .sp  .sp
2458  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern call (either by number or by
2459  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
2460  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2461  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2462  relative, as in these examples:  relative, as in these examples:
2463  .sp  .sp
# Line 2177  matches "sense and sensibility" and "res Line 2477  matches "sense and sensibility" and "res
2477  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
2478  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2479  .P  .P
2480  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2481  group. That is, once it has matched some of the subject string, it is never  groups. That is, once a subroutine has matched some of the subject string, it
2482  re-entered, even if it contains untried alternatives and there is a subsequent  is never re-entered, even if it contains untried alternatives and there is a
2483  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2484  .P  subroutine call revert to their previous values afterwards.
2485  When a subpattern is used as a subroutine, processing options such as  .P
2486  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2487  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2488    different calls. For example, consider this pattern:
2489  .sp  .sp
2490    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2491  .sp  .sp
# Line 2249  description of the interface to the call Line 2550  description of the interface to the call
2550  documentation.  documentation.
2551  .  .
2552  .  .
2553    .\" HTML <a name="backtrackcontrol"></a>
2554  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2555  .rs  .rs
2556  .sp  .sp
# Line 2264  a backtracking algorithm. With the excep Line 2566  a backtracking algorithm. With the excep
2566  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2567  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2568  .P  .P
2569  If any of these verbs are used in an assertion subpattern, their effect is  If any of these verbs are used in an assertion or in a subpattern that is
2570  confined to that subpattern; it does not extend to the surrounding pattern.  called as a subroutine (whether or not recursively), their effect is confined
2571  Note that assertion subpatterns are processed as anchored at the point where  to that subpattern; it does not extend to the surrounding pattern, with one
2572  they are tested.  exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2573    a successful positive assertion \fIis\fP passed back when a match succeeds
2574    (compare capturing parentheses in assertions). Note that such subpatterns are
2575    processed as anchored at the point where they are tested. Note also that Perl's
2576    treatment of subroutines is different in some cases.
2577  .P  .P
2578  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2579  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2580  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2581  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. A name is any sequence of
2582  are two kinds:  characters that does not include a closing parenthesis. If the name is empty,
2583    that is, if the closing parenthesis immediately follows the colon, the effect
2584    is as if the colon were not there. Any number of these verbs may occur in a
2585    pattern.
2586    .P
2587    PCRE contains some optimizations that are used to speed up matching by running
2588    some checks at the start of each match attempt. For example, it may know the
2589    minimum length of matching subject, or that a particular character must be
2590    present. When one of these optimizations suppresses the running of a match, any
2591    included backtracking verbs will not, of course, be processed. You can suppress
2592    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2593    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2594    pattern with (*NO_START_OPT).
2595    .P
2596    Experiments with Perl suggest that it too has similar optimizations, sometimes
2597    leading to anomalous results.
2598    .
2599  .  .
2600  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2601  .rs  .rs
2602  .sp  .sp
2603  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2604    followed by a name.
2605  .sp  .sp
2606     (*ACCEPT)     (*ACCEPT)
2607  .sp  .sp
2608  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2609  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2610  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is  subroutine, only that subpattern is ended successfully. Matching then continues
2611  captured. (This feature was added to PCRE at release 8.00.) For example:  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2612    far is captured. For example:
2613  .sp  .sp
2614    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2615  .sp  .sp
2616  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2617  the outer parentheses.  the outer parentheses.
2618  .sp  .sp
2619    (*FAIL) or (*F)    (*FAIL) or (*F)
2620  .sp  .sp
2621  This verb causes the match to fail, forcing backtracking to occur. It is  This verb causes a matching failure, forcing backtracking to occur. It is
2622  equivalent to (?!) but easier to read. The Perl documentation notes that it is  equivalent to (?!) but easier to read. The Perl documentation notes that it is
2623  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2624  Perl features that are not present in PCRE. The nearest equivalent is the  Perl features that are not present in PCRE. The nearest equivalent is the
# Line 2305  callout feature, as for example in this Line 2629  callout feature, as for example in this
2629  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2630  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2631  .  .
2632    .
2633    .SS "Recording which path was taken"
2634    .rs
2635    .sp
2636    There is one verb whose main purpose is to track how a match was arrived at,
2637    though it also has a secondary use in conjunction with advancing the match
2638    starting point (see (*SKIP) below).
2639    .sp
2640      (*MARK:NAME) or (*:NAME)
2641    .sp
2642    A name is always required with this verb. There may be as many instances of
2643    (*MARK) as you like in a pattern, and their names do not have to be unique.
2644    .P
2645    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2646    path is passed back to the caller via the \fIpcre_extra\fP data structure, as
2647    described in the
2648    .\" HTML <a href="pcreapi.html#extradata">
2649    .\" </a>
2650    section on \fIpcre_extra\fP
2651    .\"
2652    in the
2653    .\" HREF
2654    \fBpcreapi\fP
2655    .\"
2656    documentation. Here is an example of \fBpcretest\fP output, where the /K
2657    modifier requests the retrieval and outputting of (*MARK) data:
2658    .sp
2659        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2660      data> XY
2661       0: XY
2662      MK: A
2663      XZ
2664       0: XZ
2665      MK: B
2666    .sp
2667    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2668    indicates which of the two alternatives matched. This is a more efficient way
2669    of obtaining this information than putting each alternative in its own
2670    capturing parentheses.
2671    .P
2672    If (*MARK) is encountered in a positive assertion, its name is recorded and
2673    passed back if it is the last-encountered. This does not happen for negative
2674    assertions.
2675    .P
2676    After a partial match or a failed match, the name of the last encountered
2677    (*MARK) in the entire match process is returned. For example:
2678    .sp
2679        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2680      data> XP
2681      No match, mark = B
2682    .sp
2683    Note that in this unanchored example the mark is retained from the match
2684    attempt that started at the letter "X". Subsequent match attempts starting at
2685    "P" and then with an empty string do not get as far as the (*MARK) item, but
2686    nevertheless do not reset it.
2687    .
2688    .
2689  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2690  .rs  .rs
2691  .sp  .sp
2692  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2693  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2694  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2695    the verb. However, when one of these verbs appears inside an atomic group, its
2696    effect is confined to that group, because once the group has been matched,
2697    there is never any backtracking into it. In this situation, backtracking can
2698    "jump back" to the left of the entire atomic group. (Remember also, as stated
2699    above, that this localization also applies in subroutine calls and assertions.)
2700    .P
2701    These verbs differ in exactly what kind of failure occurs when backtracking
2702    reaches them.
2703  .sp  .sp
2704    (*COMMIT)    (*COMMIT)
2705  .sp  .sp
2706  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2707  does not match. Even if the pattern is unanchored, no further attempts to find  outright if the rest of the pattern does not match. Even if the pattern is
2708  a match by advancing the starting point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2709  passed, \fBpcre_exec()\fP is committed to finding a match at the current  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2710  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2711  .sp  .sp
2712    a+(*COMMIT)b    a+(*COMMIT)b
2713  .sp  .sp
2714  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2715  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2716  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2717    (*PRUNE)  match failure.
2718  .sp  .P
2719  This verb causes the match to fail at the current position if the rest of the  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2720  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  unless PCRE's start-of-match optimizations are turned off, as shown in this
2721  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2722  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2723  if there is no match to the right, backtracking cannot cross (*PRUNE).      re> /(*COMMIT)abc/
2724  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    data> xyzabc
2725  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2726  be expressed in any other way.    xyzabc\eY
2727      No match
2728    .sp
2729    PCRE knows that any match must start with "a", so the optimization skips along
2730    the subject to "a" before running the first match attempt, which succeeds. When
2731    the optimization is disabled by the \eY escape in the second subject, the match
2732    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2733    starting points.
2734    .sp
2735      (*PRUNE) or (*PRUNE:NAME)
2736    .sp
2737    This verb causes the match to fail at the current starting position in the
2738    subject if the rest of the pattern does not match. If the pattern is
2739    unanchored, the normal "bumpalong" advance to the next starting character then
2740    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2741    reached, or when matching to the right of (*PRUNE), but if there is no match to
2742    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2743    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2744    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2745    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2746    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2747  .sp  .sp
2748    (*SKIP)    (*SKIP)
2749  .sp  .sp
2750  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
2751  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
2752  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2753  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
2754    successful match. Consider:
2755  .sp  .sp
2756    a+(*SKIP)b    a+(*SKIP)b
2757  .sp  .sp
# Line 2352  effect as this example; although it woul Line 2762  effect as this example; although it woul
2762  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2763  instead of skipping on to "c".  instead of skipping on to "c".
2764  .sp  .sp
2765    (*THEN)    (*SKIP:NAME)
2766  .sp  .sp
2767  This verb causes a skip to the next alternation if the rest of the pattern does  When (*SKIP) has an associated name, its behaviour is modified. If the
2768  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2769  current alternation. Its name comes from the observation that it can be used  searched for the most recent (*MARK) that has the same name. If one is found,
2770  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2771    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2772    matching name is found, the (*SKIP) is ignored.
2773    .sp
2774      (*THEN) or (*THEN:NAME)
2775    .sp
2776    This verb causes a skip to the next innermost alternative if the rest of the
2777    pattern does not match. That is, it cancels pending backtracking, but only
2778    within the current alternative. Its name comes from the observation that it can
2779    be used for a pattern-based if-then-else block:
2780  .sp  .sp
2781    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2782  .sp  .sp
2783  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2784  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure, the matcher skips to the
2785  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2786  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2787    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2788    .P
2789    Note that a subpattern that does not contain a | character is just a part of
2790    the enclosing alternative; it is not a nested alternation with only one
2791    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2792    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2793    pattern fragments that do not contain any | characters at this level:
2794    .sp
2795      A (B(*THEN)C) | D
2796    .sp
2797    If A and B are matched, but there is a failure in C, matching does not
2798    backtrack into A; instead it moves to the next alternative, that is, D.
2799    However, if the subpattern containing (*THEN) is given an alternative, it
2800    behaves differently:
2801    .sp
2802      A (B(*THEN)C | (*FAIL)) | D
2803    .sp
2804    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2805    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2806    because there are no more alternatives to try. In this case, matching does now
2807    backtrack into A.
2808    .P
2809    Note also that a conditional subpattern is not considered as having two
2810    alternatives, because only one is ever used. In other words, the | character in
2811    a conditional subpattern has a different meaning. Ignoring white space,
2812    consider:
2813    .sp
2814      ^.*? (?(?=a) a | b(*THEN)c )
2815    .sp
2816    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2817    it initially matches zero characters. The condition (?=a) then fails, the
2818    character "b" is matched, but "c" is not. At this point, matching does not
2819    backtrack to .*? as might perhaps be expected from the presence of the |
2820    character. The conditional subpattern is part of the single alternative that
2821    comprises the whole pattern, and so the match fails. (If there was a backtrack
2822    into .*?, allowing it to match "b", the match would succeed.)
2823    .P
2824    The verbs just described provide four different "strengths" of control when
2825    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2826    next alternative. (*PRUNE) comes next, failing the match at the current
2827    starting position, but allowing an advance to the next character (for an
2828    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2829    than one character. (*COMMIT) is the strongest, causing the entire match to
2830    fail.
2831    .P
2832    If more than one such verb is present in a pattern, the "strongest" one wins.
2833    For example, consider this pattern, where A, B, etc. are complex pattern
2834    fragments:
2835    .sp
2836      (A(*COMMIT)B(*THEN)C|D)
2837    .sp
2838    Once A has matched, PCRE is committed to this match, at the current starting
2839    position. If subsequently B matches, but C does not, the normal (*THEN) action
2840    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2841    overrides.
2842  .  .
2843  .  .
2844  .SH "SEE ALSO"  .SH "SEE ALSO"
2845  .rs  .rs
2846  .sp  .sp
2847  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2848  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3).
2849  .  .
2850  .  .
# Line 2388  Cambridge CB2 3QH, England. Line 2862  Cambridge CB2 3QH, England.
2862  .rs  .rs
2863  .sp  .sp
2864  .nf  .nf
2865  Last updated: 30 September 2009  Last updated: 29 November 2011
2866  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2867  .fi  .fi

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